Polymer electrolyte and nonaqueous battery containing the same

ABSTRACT

A nonaqueous battery, such as a lithium ion battery, is formed from a polymer electrolyte comprising: a vinylidene fluoride copolymer comprises 90 to 97 wt. % of vinylidene fluoride monomer units and 3 to 10 wt. % of units of at least one monomer copolymerizable with the vinylidene fluoride monomer and has an inherent viscosity of 1.5 to 10 dl/g. The polymer electrolyte stably retains the nonaqueous electrolytic solution in a large amount and has excellent strength in this state.

TECHNICAL FIELD

The present invention relates to a nonaqueous battery, particularly alithium ion battery, and a nonaqueous battery containing theelectrolyte.

BACKGROUND ART

The development of electronic technology in recent years is remarkable,and various apparatus and devices have been reduced in size and weight.Accompanying the reduction in size and weight of such electronicapparatus and devices, there has been a remarkably increasing demand forreduction in size and weight of a battery as a power supply for suchelectronic apparatus and devices. As a battery capable of providing alarge energy at small volume and weight, a nonaqueous secondary batteryusing lithium has been used as a power source for principallysmall-sized electronic appliances, such as portable telephone sets,personal computers and video cam coders, used at home. For the purposeof providing the lithium nonaqueous secondary battery with increasedshape latitude, e.g., formation into a very small thickness on the orderof 0.5 mm, extensive development work has been made on polymerelectrolyte batteries.

A polymer electrolyte containing no electrolytic solution hardlysatisfies properties required for application to batteries because of,e.g., low ionic conductivity and small battery discharge capacity. Incontrast thereto, a polymer gel electrolyte containing electrolyticsolution has called an attention because of a high ionic conductivity.As such a polymer electrolyte, U.S. Pat. No. 5,296,318 has reported apolymer electrolyte using a copolymer of vinylidene fluoride with 8 to25 wt. % of hexafluoropropylene. Further, as a technique forimpregnating the copolymer with an increased amount of electrolyticsolution, U.S. Pat. No. 5,456,000 has disclosed a technique of mixingthe copolymer with a plasticizer, then extracting the plasticizer andthen effecting the impregnation with a nonaqueous electrolytic solution.According to this technique, it is possible to effect the impregnationwith a large amount of electrolytic solution, but such impregnation witha large amount of electrolytic solution is accompanied with a problem oflosing a shape latitude, such as the formation into a very smallthickness. Further, as the technique essentially involves a step ofextracting the plasticizer, the productivity becomes inferior. Further,complete extraction of a plasticizer is difficult, and a portion of theplasticizer remaining in the polymer electrolyte is liable to exert anadverse effect to the battery prepared by using the electrolyte.

In order to obtain a polymer electrolyte battery having a high shapelatitude, it is essential to provide a polymer gel electrolyte capableof containing a large amount of electrolytic solution so as to enhancethe ionic conductivity and yet exhibiting a large strength. However, thestrength of a gel is lowered at a larger content of electrolyticsolution, so that it has been impossible to satisfy a gel strength and acontent of electrolytic solution in combination, and no polymer gelelectrolyte suitable for providing a polymer electrolyte battery havinga high shape latitude has been known.

In order to increase the gel strength, it is considered important toprovide an enhanced modulus of elasticity to the gel. Factorscontrolling the elasticity modulus of a gel have been generally obscureexcept that a higher polymer concentration provides a higher elasticitymodulus (but this results in a lower content of electrolytic solution inthe polymer electrolyte and is thus not practical), and it has beenreported that an increase in polymer molecular weight does not result ina change in elasticity modulus with respect to κ carrageenans gel byRochas, C. et al, Carbohydrate Polymers, 12, 255-266 (1990). In thisway, as general guiding principles for enhancing the gel strength, nonehave been known except for relying on a higher polymer concentration.Accordingly, a practical polymer electrolyte capable of beingimpregnated with a large amount of nonaqueous electrolytic solution andyet having an excellent strength, has not been known.

Further, in the case of being impregnated with a large amount ofnonaqueous electrolytic solution, it is necessary to stably retain thesolution and prevent the solution from leaking out of the polymerelectrolyte. If the nonaqueous electrolytic solution cannot be stablyretained and a large amount of leakage thereof is caused, it becomesimpossible to obviate damages and deterioration of electrical propertiesof apparatus and devices surrounding the battery.

DISCLOSURE OF INVENTION

The present invention aims at providing a polymer electrolyte capable ofbeing impregnated with a large amount of nonaqueous electrolyticsolution and stably retaining the electrolytic solution and yetexhibiting excellent strength, and further a nonaqueous battery having alarge shape latitude by using the polymer electrolyte.

According to the inventors' study for accomplishing the above objects,it has been found very preferable to use a polymer electrolyte,comprising: a vinylidene fluoride copolymer and a nonaqueouselectrolytic solution, wherein the vinylidene fluoride copolymercomprises 80 to 97 wt. % of vinylidene fluoride monomer units and 3 to20 wt. % of units of at least one monomer copolymerizable withvinylidene fluoride monomer and has an inherent viscosity of 1.5 to 10dl/g. Herein, “inherent viscosity” is used as a measure of polymermolecular weight and refers to a logarithmic viscosity number asmeasured at 30° C. of a solution formed by dissolving 4 g of a polymerresin in 1 liter of N,N-dimethylformamide.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view in a thickness-wise direction of a nonaqueousbattery using a polymer electrolyte according to the present invention.

EMBODIMENTS OF THE INVENTION

The polymer electrolyte according to the present invention comprises avinylidene fluoride copolymer and a nonaqueous electrolytic solution,wherein the vinylidene fluoride copolymer comprises 80 to 97 wt. % ofvinylidene fluoride monomer units and 3 to 20 wt. % of units of one orplural species of monomer copolymerizable with vinylidene fluoridemonomer and has an inherent viscosity of 1.5 to 10 dl/g, preferably 1.7to 7 dl/g.

As a polymer matrix capable of keeping electrochemical stability whilecontaining an electrolytic solution in a lithium nonaqueous secondarybattery, it is possible to suitably use, e.g., a vinylidenefluoride-hexafluoropropylene or a vinylidenefluoride-trifluorochloroethylene copolymer. If the content of themonomer other than vinylidene fluoride is below 3 wt. %, the content ofthe electrolytic solution cannot be increased, and if the vinylidenefluoride monomer content is below 80 wt. %, the liquid-retentivity ofthe polymer electrolyte is lowered in the case of retaining a largeamount of electrolytic solution, thus being liable to cause theexudation of the electrolytic solution, so that the vinylidene fluoridemonomer unit content should be in the range of 80 wt. % to 97 wt. %. Aternary copolymer comprising hexafluoropropylene andtrifluorochloroethylene in a total amount of 3-20 wt. % in addition to80-97 wt. % of vinylidene fluoride, may also be preferably used.

The vinylidene copolymer may be produced through a process, such assuspension polymerization, emulsion polymerization or solutionpolymerization, and the polymerization process need not be particularlyrestricted. For the purpose of, e.g., improving the properties of theresultant polymer electrolyte, it is possible to adjust a method ofadding copolymerizable monomers, a polymerization temperature, etc. Asfor the method of adding copolymerizable monomers, e.g., in the case ofcopolymerization of vinylidene fluoride and hexafluoropropylene, thepolymerization of vinylidene fluoride and hexafluoropropylene chargedsimultaneously can provide a polymer resulting in a polymer electrolytecapable of exhibiting a higher strength and a higher liquid-retentivityand is therefore advantageous than in the case of polymerization ofvinylidene fluoride and hexafluoropropylene charged in division orcontinuously. As for the polymerization temperature, a highertemperature provides a polymer resulting in a polymer electrolyteexhibiting a higher liquid-retentivity and is therefore advantageous. Atemperature of 25° C. or higher is generally suitable. In the case ofsuspension polymerization, for example, a temperature of 25° C.-50° C.is suitable at the initial stage but it is also preferred to raise thetemperature up to ca. 80° C. at a later stage. Further, in the case ofemulsion polymerization, it is possible to raise the temperature up toca. 150° C. from the initial stage.

A vinylidene fluoride copolymer of a high liquid-retentivity obtained bysuch a relatively high polymerization temperature is characterized by anincrease in abnormal linkage or different-type linkage (head-head ortail-tail linkage) at vinylidene fluoride sites formed of successive oradjacent vinylidene fluoride polymerized units as confirmed by NMR. Suchan abnormal linkage content should preferably be at last 3% ofvinylidene fluoride sites.

Incidentally, U.S. Pat. No. 5,296,318 has disclosed the use of avinylidene fluoride copolymer having a relatively low vinylidenefluoride monomer unit content by including 8-25 wt. % ofhexafluoropropylene so as to provide an increased impregnation contentof nonaqueous electrolytic solution. In the present invention, however,even at a higher vinylidene fluoride content of, e.g., 93 wt. %, a highimpregnation content of nonaqueous electrolytic solution can be attainedtogether with a remarkably improved retentivity of non-aqueouselectrolytic solution (see Examples 1, 4, etc., described later). Whilethe reason therefor is not clear as yet, the effect is understood as aneffect accompanying an increased inherent viscosity of at least 1.5dl/g, i.e., an increased molecular weight, of the copolymer (seeExamples and Comparative Examples described later).

If the polymer has an inherent viscosity of below 1.5 dl/g, theresultant polymer electrolyte is caused to have a weak strength at ahigh electrolytic solution content region usable as a battery, and ashort circuit between the positive and negative electrodes is causedwhen a thin battery is formed and folded, so that the polymerelectrolyte is difficult to use from a viewpoint of dynamical strength.There is observed a tendency that a higher inherent viscosity of polymerresults in a polymer electrolyte exhibiting a higher strength, but abovea certain inherent viscosity, the strength tends to be saturated.Moreover, an inherent viscosity in excess of 10 dl/g results in aproblem in respect of productivity of the vinylidene fluoride that itbecomes difficult to form a thick solution thereof in a volatilesolvent.

More specifically, a vinylidene fluoride copolymer comprising 80-97 wt.% of vinylidene fluoride monomer units and 3-20 wt. % of one or pluralspecies of monomer copolymerizable with vinylidene fluoride and havingan inherent viscosity of 1.5-10 dl/g, allows easy formation of a gelfilm, which shows a good liquid-retentivity and a large film strengtheven in a gel state containing an electrolytic solution in an amount aslarge as 300 wt. % of the polymer (i.e., an electrolytic solutioncontent in the gel of 75 wt. %), so that it is suitably used in anonaqueous battery comprising a polymer electrolyte. The polymerelectrolyte according to the present invention can be used in a state ofcontaining an electrolytic solution at a large content of ordinarily 50wt. % to 85 wt. %.

There is observed a tendency that a polymer electrolyte exhibits ahigher lithium ionic conductivity at a higher electrolytic solutioncontent. For example, the above-mentioned U.S. Pat. No. 5,296,318discloses that gels having electrolytic solution contents in the rangeof 20 wt. %-70 wt. %, substantially 40-60 wt. %, show lithium ionconductivities ranging from 10⁻⁵ S/cm to 10⁻³ S/cm. Accordingly, it isensured that the polymer electrolyte gel according to the presentinvention capable of exhibiting a higher electrolytic solution contentof 50-85 wt. % in the polymer electrolyte exhibits a level of ionicconductivity sufficient for function as an actual battery material.Particularly, the polymer electrolyte according to the present inventionshows a sufficient gel strength in a state of containing an electrolyticsolution in a proportion as high as 85 wt. % as shown in Examplesdescribed later.

Examples of the monomer copolymerizable with vinylidene fluoride monomermay include: hydrocarbon monomers, such as ethylene and propylene;fluorine-containing monomers, such as vinyl fluoride, trifluoroethylene,trifluorochloroethylene, tetrafluoroethylene, hexafluoropropylene, andfluoroalkyl vinyl ether; carboxyl group-containing monomers, such asmonomethyl maleate and monomethyl citraconate; and epoxygroup-containing vinyl monomers, such as ally glycidyl ether andglycidyl crotonate, but these are not restrictive. It is howeverpreferred to use vinylidene fluoride copolymers containinghexafluoropropylene and/or trifluoroethylene among the above.

The nonaqueous electrolytic solution constituting the gel-form polymerelectrolyte according to the present invention together with a matrix ofthe above-mentioned vinylidene fluoride copolymer may, for example, beobtained by dissolving an electrolyte, such as a lithium salt, in aproportion of 5-30 wt. parts in 100 wt. parts of a nonaqueous solvent(organic solvent).

The electrolytes may for example include: LiPF₆, LiAsF₆, LiClO₄, LiBF₄,LiCl, LiBr, LiCH₃SO₃, LiCF₃SO₃, LiN(CF₃SO₂)₂ and LiC(CF₃SO₂)₃. Theorganic solvent for the electrolyte may for example include: propylenecarbonate, ethylene carbonate, 1,2-diethoxyethane, 1,2-diethoxyethane,dimethyl carbonate, methyl ethyl carbonate, γ-butylolactone, methylpropionate, ethyl propionate, and solvent mixtures of these, but theseare not restrictive.

The polymer electrolyte according to the present invention may be formedfrom the above-mentioned vinylidene fluoride copolymer resin (or amixture thereof with another resin) and nonaqueous electrolyticsolution, e.g., in the following manner. First, an electrolyte isdissolved in an organic solvent to form an electrolytic solution in amanner as described above. Then, a vinylidene fluoride resin isdissolved in a volatile organic solvent to form a solution, which isblended with the above nonaqueous electrolytic solution. Further, via astep of vaporizing the above-mentioned volatile organic solvent, apolymer electrolyte in the form of a film is obtained. The volatileorganic solvent used in this instance may preferably be one which isreadily volatile by having a high vapor pressure at a relatively lowtemperature and can well dissolve the vinylidene fluoride copolymer.Tetrahydrofuran, methyltetrahydrofuran, acetone, methyl ethyl ketone,1,3-dioxalan, cyclohexanone, etc., may be used, but these are notrestrictive.

Further, propylene carbonate, ethylene carbonate, dimethyl carbonate,etc., frequently used as an organic solvent for dissolving anelectrolyte can per se be used as a solvent for the vinylidene fluoridecopolymer, so that it is possible to form a polymer electrolyte withoutusing a volatile organic solvent as described above. In this instance,it is possible to first dissolve a vinylidene fluoride copolymer in anorganic solvent to form a solution and then add an electrolyte theretofor further dissolution, or to dissolve a vinylidene fluoride copolymerand an electrolyte simultaneously in an organic solvent. The resultantsolution containing the vinylidene fluoride copolymer and theelectrolyte is cooled to room temperature for gelation, thereby forminga film structure comprising a polymer electrolyte in the form of a film.

A basic structure of a nonaqueous battery using a polymer electrolyteaccording to the present invention may be obtained as shown in asectional view of FIG. 1 by disposing a generally sheet-form polymerelectrolyte in a sandwiched form between a pair of a positive electrode2 (2 a: electroconductive substrate, 2 b: positive composite electrodelayer) and a negative electrode 3 (3 a: electroconductive substrate, 3b: negative composite electrode layer).

In the case of a lithium ion battery taken for example, the sheet-formpolymer electrolyte 1 may preferably have a thickness of ca. 2-1000 μm,particularly ca. 10-200 μm, and it is preferred to use a nonaqueouselectrolytic solution for impregnation in a proportion of 10-1000 wt.parts, particularly 100-500 wt. parts, for 100 wt. parts of thevinylidene fluoride copolymer.

Further, in order to provide an improved heat resistance, it is possibleto crosslink the polymer electrolyte. As chemical crosslinking means, itis suitable to apply a vulcanization method for fluoroelastomer obtainedby copolymerization of vinylidene fluoride with another monomer. Morespecifically, the crosslinking may be performed by adding a polyamine, apolyol or a polyfunctional crosslinking agent, and a radical generatingagent.

Suitable examples of the polyamine used for chemical crosslinking mayinclude dibutylamine, piperidine, diethylcyclohexylamine,hexamethylenediamine, hexamethylenediamine carbamate,N,N′-dicinnamilidene-1,6-hexanediamine, and 4,4′-bis(aminocyclohexyl)metacarbamate, but these are not restrictive.

Suitable examples of the polyol may include2,2-bis(4-hydroxyphenyl)propane,2,2-bis(4-hydroxyphenyl)hexafluoropropane, hydroquinone, and4,4′-dihydroxydiphenylmethane, but these are not restrictive.

Suitable examples of the polyfunctional crosslinking agent having anunsaturated bond may include: divinylbenzene, ethylene glycoldimethacrylate, triethylene glycol dimethacrylate, tetraethylene glycoldimethacrylate, 1,3-butylene glycol dimethacrylate, propylene glycoldimethacrylate, 1,4-butanediol dimethacrylate, 1,6-hexanedioldimethacrylate, neopentyl glycol dimethacrylate, allyl methacrylate,allyl acrylate, 2-hydroxy-1,3-dimethacryloxypropane, bisphenoldimethacrylates, alicyclic dimethacrylates, diacryl isocyanurate,trimethylolpropane trimethacrylate, triacrylformal, triacrylisocyanurate, triallyl isocyanurate, aliphatic triacrylates,pentaerythritol tetramethacrylate, pentaerythritol tetraacrylate, andaliphatic tetraacrylates, but these are not restrictive.

As the radical generator, various organic peroxides may be usedincluding, as suitable examples, dialkyl peroxides, such as di-t-butylperoxide; diacyl peroxides, such as benzoyl peroxide; peroxyketals, suchas 2,5-dimethyl-di(t-butylperoxy)hexane; and di-n-peroxydicarbonates,but these are not restrictive.

Further, in addition to the above-mentioned polyamine, polyol,polymerizable crosslinking agent and radical generator, it is alsopossible to add, as a vulcanization accelerator, a compound whichpromotes the defluorination of vinylidene fluoride but per se is notreadily added. Examples of the vulcanization accelerator may includeorganic phosphonium salts and quaternary ammonium salts represented byR₄P⁺X⁻ and R₄N⁺X⁻.

As another method of crosslinking the polymer electrolyte, theirradiation with electron beam or γ-rays may suitably be adopted forintroducing the crosslinking structure. The radiation dose in thisinstance may suitably be on the order of 10-500 kGy. In order to enhancethe radiation crosslinking effect, it is also suitable to add apolymerizable crosslinking agent having an unsaturated bond as mentionedabove in the solid polymer electrolyte in advance.

The positive electrode 2 and the negative electrode 3 may be obtained byforming a positive composite electrode layer 2 b and a negativecomposite electrode layer 3 b in thicknesses of, e.g., 10-1000 μm on,e.g., one surface each of electroconductive substrates 2 a and 3 acomprising a metal foil or metal net comprising iron, stainless steel,copper, aluminum, nickel, titanium, etc. in thicknesses of 5-100 μm,e.g., 5-20 μm in case of small-sized devices.

The positive composite electrode layer 2 b and 3 b may for example beobtained by applying and drying an electrode-forming slurry compositionformed, e.g., by dispersing 1-20 wt. parts of powdery electrodematerials (positive or negative electrode active substance, andoptionally added electroconductivity imparting agent and otheradditives) in 100 wt. parts of a solution of a vinylidene fluoridecopolymer in general inclusive of the above-mentioned vinylidenefluoride copolymer and an electrolytic solution in a volatile organicsolvent.

Preferred active substances for lithium ion secondary batteries mayinclude: for positive electrodes, complex metal chalogenides representedby a general formula of LiMY₂ (wherein M denotes at least one species oftransition metals, such as Co, Ni, Fe, Mn, Cr and V; and Y denotes achalcogen such as O or S), particularly complex metal oxides asrepresented by LiNi_(x)Co_(1-x)O₂ (0≦x≦1) and complex metal oxideshaving a spinel structure, such as LiMn₂O₄.

Active substances for negative electrodes may include: carbonaceousmaterials, such as graphite, activated carbon, calcined and carbonizedproducts of phenolic resin and pitch, and coconut shell-based activatedcarbon, and metal oxides, such as GeO, GeO₂, SnO, SnO₂, PbO, PbO₂, SiO,SiO₂, etc., and complex metal oxides of these.

The thus-formed laminated sheet-form battery structure shown in FIG. 1may be, as desired, further laminated as by winding or folding toprovide an increased electrode area per unit volume, and subjected to atreatment, such as enclosure within a relatively simple container andformation of lead electrodes, to provide a non-aqueous battery having anentire structure of, e.g., a rectangle, a cylinder, a coin or apaper-sheet.

EXAMPLES

Hereinbelow, the present invention will be described more specificallywith reference to Examples and Comparative Examples.

Incidentally, abnormal linkage contents in vinylidene fluoride polymersdescribed in Examples below were measured according to the followingmethod.

[Measurement of Abnormal Linkage Content]

The abnormal linkage content of a vinylidene fluoride polymer isdetermined from diffraction peaks of its ¹⁹F-NMR spectrum.

More specifically, ca. 5 mg of a vinylidene fluoride polymer sample isdissolved in a solvent mixture of 0.4 ml of dimethylformamide (DMF) and0.1 ml of deuterium dimethylformamide (DMF-d₇) as a solvent for NMRmeasurement, and the resultant solution is subjected to the ¹⁹F-NMRmeasurement at room temperature.

A resultant ¹⁹F-NMR spectrum generally exhibits plural peaks.

Among these peaks, peaks at −113.5 ppm and −115.9 ppm with reference toCFCl₃ are identified as peaks attributable to the abnormal linkage.

The abnormal linkage content is determined as follows. The areas ofpeaks in the range of −90 ppm to −115.9 ppm are respectively measuredand summed up to provide a total area S₀. On the other hand, the areasof the peaks at −113.5 ppm and −115.9 ppm are denoted by S₁ and S₂,respectively. Then, the abnormal linkage content is calculated accordingto the following formula:

Abnormal linkage content=[{(S ₁ +S ₂)/2}/S ₀]×100(%)

(Preparation of Vinylidene Fluoride Copolymers) Polymer PreparationExample 1

Into an autoclave having an inner volume of 2 liters, 1075 g ofdeionized water, 0.42 g of methyl cellulose, 2.9 g of diisopropylperoxydicarbonate, 378 g of vinylidene fluoride and 42 g ofhexafluoropropylene were charged and subjected to 10 hours of suspensionpolymerization at 29° C. After completion of the polymerization, thepolymerization slurry was dewatered, washed with water and dried at 80°C. for 20 hours to obtain a polymer powder. The polymerization yield was89 wt. %, and the resultant polymer showed an inherent viscosity of 1.7dl/g. As a result of ¹⁹F-NMR analysis, the polymer exhibited a weightratio of vinylidene fluoride to hexafluoropropylene of 93:7, and anabnormal linkage (head-head or tail-tail linkage) content at vinylidenefluoride sites of 3.6%.

Polymer Preparation Example 2

Into an autoclave having an inner volume of 2 liters, 1036 g ofdeionized water, 0.40 g of methyl cellulose, 2.8 g of diisopropylperoxydicarbonate, 376 g of vinylidene fluoride and 4 g oftrichlorofluoroethylene were charged and subjected to 16 hours ofsuspension polymerization at 28° C. while adding 20 g oftrifluorochloroethylene in division of 1 g each at every 30 minutes from2 hours after the start of the polymerization. After completion of thepolymerization, the polymerization slurry was dewatered, washed withwater and dried at 80° C. for 20 hours to obtain a polymer powder. Thepolymerization yield was 86 wt. %, and the resultant polymer showed aninherent viscosity of 1.8 dl/g. As a result of ¹⁹F-NMR analysis, thepolymer exhibited a weight ratio of vinylidene fluoride totrifluorochloroethylene of 95:5.

Polymer Preparation Example 3

Into an autoclave having an inner volume of 2 liters, 1075 g ofdeionized water, 0.42 g of methyl cellulose, 2.9 g of diisopropylperoxydicarbonate, 365 g of vinylidene fluoride, 30 g ofhexafluoropropylene and 25 g of trifluorochloroethylene were charged andsubjected to 11 hours of suspension polymerization at 29° C. Aftercompletion of the polymerization, the polymerization slurry wasdewatered, washed with water and dried at 80° C. for 20 hours to obtaina polymer powder. The polymerization yield was 90 wt. %, and theresultant polymer showed an inherent viscosity of 1.8 dl/g. As a resultof ¹⁹F-NMR analysis, the polymer exhibited weight ratios of vinylidenefluoride to hexafluoropropylene to trifluorochloroethylene of 90:5:5.

Polymer Preparation Example 4

Into an autoclave having an inner volume of 2 liters, 1140 g ofdeionized water, 0.29 g of methyl cellulose, 4.4 g of diisopropylperoxydicarbonate, 326 g of vinylidene fluoride and 52 g ofhexafluoropropylene were charged and subjected to 11 hours of suspensionpolymerization at 29° C. while adding 204 g of vinylidene fluoride indivision of 17 g each at every 25 minutes from 3 hours after the startof the polymerization. After completion of the polymerization, thepolymerization slurry was dewatered, washed with water and dried at 80°C. for 20 hours to obtain a polymer powder. The polymerization yield was90 wt. %, and the resultant polymer showed an inherent viscosity of 1.7dl/g. As a result of ¹⁹F-NMR analysis, the polymer exhibited a weightratio of vinylidene fluoride to hexafluoropropylene of 93:7.

Polymer Preparation Example 5

Into an autoclave having an inner volume of 2 liters, 1075 g ofdeionized water, 0.21 g of methyl cellulose, 2.9 g of diisopropylperoxydicarbonate, 378 g of vinylidene fluoride and 42 g ofhexafluoropropylene were charged and subjected to suspensionpolymerization at 29° C. for 6 hours. When the pressure was lowered by0.3 MPa from the pressure at the start of the polymerization, thetemperature was raised to 40° C., and polymerization was performed forfurther 6 hours. After completion of the polymerization, thepolymerization slurry was dewatered, washed with water and dried at 80°C. for 20 hours to obtain a polymer powder. The polymerization yield was84 wt. %, and the resultant polymer showed an inherent viscosity of 1.7dl/g. As a result of ¹⁹F-NMR analysis, the polymer exhibited a weightratio of vinylidene fluoride to hexafluoropropylene of 93:7, and anabnormal linkage content at vinylidene fluoride sites of 4.0%.

Polymer Preparation Example 6

Into an autoclave having an inner volume of 2 liters, 1075 g ofdeionized water, 0.21 g of methyl cellulose, 2.9 g of diisopropylperoxydicarbonate, 370 g of vinylidene fluoride and 50 g ofhexafluoropropylene were charged and subjected to 18 hours of suspensionpolymerization at 29° C. After completion of the polymerization, thepolymerization slurry was dewatered, washed with water and dried at 80°C. for 20 hours to obtain a polymer powder. The polymerization yield was89 wt. %, and the resultant polymer showed an inherent viscosity of 1.9dl/g. As a result of ¹⁹F-NMR analysis, the polymer exhibited a weightratio of vinylidene fluoride to hexafluoropropylene of 91:9.

Polymer Preparation Example 7

Into an autoclave having an inner volume of 2 liters, 1075 g ofdeionized water, 0.21 g of methyl cellulose, 2.1 g of diisopropylperoxydicarbonate, 378 g of vinylidene fluoride and 42 g ofhexafluoropropylene were charged and subjected to 8 hours of suspensionpolymerization at 29° C. After completion of the polymerization, thepolymerization slurry was dewatered, washed with water and dried at 80°C. for 20 hours to obtain a polymer powder. The polymerization yield was88 wt. %, and the resultant polymer showed an inherent viscosity of 2.5dl/g. As a result of ¹⁹F-NMR analysis, the polymer exhibited a weightratio of vinylidene fluoride to hexafluoropropylene of 93:7.

Example 1

10 g of the polymer obtained in Polymer Preparation Example 1 and 100 gof a mixture solution of ethylene carbonate, propylene carbonate andtetrahydrofuran (in mixing weight ratios of 15:15:70) were blended toform a solution. The solution was cast, and the tetrahydrofuran wasremoved therefrom by air drying to form a ca. 80 μm-thick gel-form filmcontaining the ethylene carbonate and propylene carbonate at ca. 75 wt.%. (As a result of weighing of the resultant gel-form film, a weightloss corresponding to the used tetrahydrofuran was confirmed.)

According to ASTM D882, a test piece was cut out from the gel-form filmin a test length of 20 mm and a test width of 10 mm and subjected tomeasurement of a tensile strength at a tensile speed of 100 mm/min. byusing TENSILON UTM-III-100 (made by TOYO BALDWIN K.K.), whereby a valueof 2.54 MPa was obtained.

Example 2

A ca. 100 μm-thick gel-form film containing ca. 75 wt. % of ethylenecarbonate and propylene carbonate was obtained in the same manner as inExample 1 except for using the polymer obtained in Polymer PreparationExample 2. As a result of weighing of the gel-form film, a weight losscorresponding to the used tetrahydrofuran was confirmed.

As a result of a tensile strength measurement of the gel-form film inthe same manner as in Example 1, a value of 2.01 MPa was obtained.

Example 3

A ca. 100 μm-thick gel-form film containing ca. 75 wt. % of ethylenecarbonate and propylene carbonate was obtained in the same manner as inExample 1 except for using the polymer obtained in Polymer PreparationExample 3. As a result of weighing of the gel-form film, a weight losscorresponding to the used tetrahydrofuran was confirmed.

As a result of a tensile strength measurement of the gel-form film inthe same manner as in Example 1, a value of 1.54 MPa was obtained.

Comparative Example 1

A ca. 100 μm-thick-gel-form film containing ca. 75 wt. % of ethylenecarbonate and propylene carbonate was obtained in the same manner as inExample 1 except for using as the polymer “KYNAR 2801” (morespecifically “Kynar FLEX 2801” made by Atochem Co., a vinylidenefluoride/hexafluoropropylene copolymer; a vinylidenefluoride/hexafluoropropylene nominal weight ratio=88/12. According tothe inventors' measurement, vinylidene fluoride/hexafluoropropyleneweight ratio=90/10 based on NMR analysis and inherent viscosity=1.2dl/g). As a result of weighing of the gel-form film, a weight losscorresponding to the used tetrahydrofuran was confirmed.

As a result of a tensile strength measurement of the gel-form film inthe same manner as in Example 1, only a weak strength of 0.76 MPa wasobtained.

Example 4

In a nitrogen atmosphere having a dew point of below −70° C., 10 goofthe polymer obtained in Polymer Preparation Example 1 and 5 g of LiPF₆were blended with 100 g of a solution mixture of ethylene carbonate,propylene carbonate and tetrahydrofuran (in mixing wt. ratios of15:15:70) to form a solution. The solution was cast, and thetetrahydrofuran was removed by air drying to form a ca. 80 μm-thickgel-form polymer electrolyte film. As a result of weighing of theresultant gel-form polymer electrolyte film, a weight loss correspondingto the used tetrahydrofuran was confirmed.

The gel-form polymer electrolyte exhibited little exudation of theelectrolytic solution and was found to be soft, stretchable and strongas a result of pulling by hands. A test piece of 50 mm×50 mm was cut outfrom the gel-form polymer electrolyte film and, after weighing, storedfor 2 weeks at −18° C., followed by restoration to room temperature,light wiping of the film surface to remove the electrolytic solution atthe film surface and weighing to determine a percentage weight loss dueto exudation for evaluating the electrolytic solution-retentivity.Herein, the percentage weight loss is given by ((weight beforestorage−weight after storage)/(weight before storage))×100, and asmaller value indicates a stabler electrolytic solution-retentivity withtime. The percentage weight loss was a small value of 0.38%, thusindicating an excellent electrolytic solution-retentivity.

Example 5

A ca. 80 μm-thick gel-form polymer electrolyte film was prepared in thesame manner as in Example 1 except for using the polymer prepared inPolymer Preparation Example 2. As a result of evaluation of electrolyticsolution-retentivity of the gel film in the same manner as in Example 4,the percentage weight loss due to exudation was as small as 0.11%, thusindicating an excellent electrolytic solution-retentivity.

Example 6

A ca. 80 μm-thick gel-form polymer electrolyte film was prepared in thesame manner as in Example 1 except for using the polymer prepared inPolymer Preparation Example 3. As a result of evaluation of electrolyticsolution-retentivity of the gel film in the same manner as in Example 4,the percentage weight loss due to exudation was as small as 0.49%, thusindicating an excellent electrolytic solution-retentivity.

Comparative Example 2

A ca. 80 μm-thick gel-form polymer electrolyte film was prepared in thesame manner as in Example 4 except for using as the polymer “KYNAR 2801”(vinylidene fluoride/hexafluoropropylene weight ratio=88/12, inherentviscosity 1.2 dl/g). As a result of weighing of the resultant gel-formpolymer electrolyte film, a weight loss corresponding to the usedtetrahydrofuran was confirmed.

The gel-form polymer electrolyte film exhibited exudation of theelectrolytic solution and a clearly inferior strength than those ofExamples 3 and 4. Further, as a result of evaluation of electrolyticsolution-retentivity in the same manner as in Example 4, the percentageweight loss due to exudation was as large as 1.50%, thus indicating aninferior electrolytic solution-retentivity.

Example 7

10 g of the polymer obtained in Polymer Preparation Example 3 wasdissolved in 90 g of tetrahydrofuran, and 0.5 g of hexamethylenediamineas a crosslinking agent and 0.5 g of carbon black as an accelerator wereadded thereto to prepare a first solution. Then, 4.5 g of LiPF₆ wasdissolved in 3 ml of a mixture solution of propylene carbonate andethylene carbonate in a volume ratio of 1:1 to prepare a secondsolution. The first solution and the second solution were blended andwell stirred for 12 hours at 50° C., followed by casting onto a glasssheet and removal of the tetrahydrofuran by air drying to obtain agel-form polymer electrolyte film. As a result of weighing of theresultant ca. 80 μm-thick gel film, a weight loss corresponding to theused tetrahydrofuran was confirmed.

As a result of tensile strength measurement of the gel-form film in thesame manner as in Example 1, a value of 3.61 MPa was obtained.

Then, as a heat resistance test, the gel-form film was hermeticallysealed up in a glass bottle and heated for 1 hour within an oven at 100°C., followed by taking out and cooling to room temperature. The gel-formfilm after the cooling retained its original shape without meltingduring the heating.

Example 8

In a nitrogen atmosphere having a dew point of below −70° C., 10 g ofthe polymer prepared in Polymer Preparation Example 1 and 5 g of LiPF₆were dissolved in 100 g of a mixture solution of propylene carbonate,ethylene carbonate and dimethyl carbonate (in mixing wt. ratios of15:15:70) to form a first solution. On the other hand, 7 g ofpolyvinylidene fluoride (“KF#1300”, made by Kureha Kagaku K.K.; inherentviscosity=1.30 dl/g) was mixed with 85 g of LiCoO₂, 8 g ofelectroconductive carbon black and 60 g of N-methyl-2-pyrrolidone, andthe resultant slurry was applied on a 10 μm-thick aluminum foil,followed by vaporization removal of the N-methyl-2-pyrrolidone to form aca. 110 μm-thick dry electrode (positive electrode). Further, 10 g ofpolyvinylidene fluoride (“#9100”, made by Kureha Kagaku K.K.; inherentviscosity=1.10 dl/g) was mixed with 90 g of a pitch-based porouscarbonaceous material and 90 g of N-methyl-2-pyrrolidone, and theresultant slurry was applied onto a 10 μm-thick copper foil, followed byvaporization removal of the N-methyl-2-pyrrolidone to form a ca. 105μm-thick dry electrode (negative electrode).

Then, the first solution was divided into equal halves which were thenseparately applied onto the active substances of the positive electrodeand the negative electrode, followed by air drying to causeevaporation-off of ca. 60 g of the dimethyl carbonate having a lowerboiling point and form a gel-form polymer electrolyte layer on thepositive and negative electrodes. The positive electrode and thenegative electrode each coated with the gel layer were laminated withtheir gel layers inside by a double roller laminator, thereby forming apaper-form battery having a total thickness of ca. 0.7 mm including abattery case.

The paper-form battery was bent at 90 deg. and, in the bent state,subjected to a charging operation according to a constantcurrent-constant voltage charging method wherein the battery was firstcharged at a current density of 1.8 mA/cm² up to a battery voltage of4.2 volts and then held at a constant voltage of 4.2 volts within atotal changing time not exceeding 3.5 hours, followed by a dischargingoperation according to a constant current discharging method wherein thebattery was discharged at a current density of 1.8 mA/cm² down to afinal voltage of 2.5 volts. In the first cycle, the battery exhibited acharging capacity of 332 mAh/g (carbon material) and a dischargecapacity of 287 mAh/g (carbon material). On further repetition of thecharge-discharge cycles, the discharge capacity at 20th-cycle was 97% ofthe capacity at the first cycle. During the cycles, the charge-dischargeoperations were smoothly performed without causing liquid leakage.

Comparative Example 3

A paper-form battery was prepared in the same manner as in Example 8except for using as the polymer “KYNAR 2801” (vinylidenefluoride/hexafluoro-propylene weight ratio=88/12, inherentviscosity=1.2). The same charge-discharge test in a bent state at 90deg. as in Example 8 was tried to be applied to the battery, whereas ashort circuit between the positive and negative electrodes was causedpresumably due to breakage of the gel electrolyte layer so that thecharging was failed.

Example 9

A ca. 100 μm-thick gel-form film containing ca. 75 wt. % of ethylenecarbonate and propylene carbonate was prepared in the same manner as inExample 1 except for using the polymer obtained in Polymer PreparationExample 4. As a result of weighing of the gel-form film, a weight wascorresponding to the used tetrahydrofuran was confirmed. As a result ofa tensile strength measurement of the gel-form film, a value of 1.68 MPawas obtained.

A ca. 80 μm-thick gel-form polymer electrolyte film was prepared in thesame manner as in Example 4 except for using the polymer obtained inPolymer Preparation Example 4. As a result of evaluation of electrolyticsolution-retentivity of the gel film in the same manner as in Example 4,the percentage weight loss due to exudation was at a small value of0.53%, thus exhibiting an excellent electrolytic solution-retentivity.

The above Examples are comparable to Examples 1 and 4 in respects ofpolymer composition and inherent viscosity. Thus, it is understood thatthe polymer of Polymer Preparation Example 1 obtained by polymerizationof the monomers charged simultaneously provided higher strength andhigher electrolytic solution-retentivity than the polymer of PolymerPreparation example 4 obtained by polymerization of the monomers chargedin division.

Example 10

A ca. 100 μm-thick gel-form film containing ca. 75 wt. % of ethylenecarbonate and propylene carbonate was prepared in the same manner as inExample 1 except for using the polymer obtained in Polymer PreparationExample 5. As a result of weighing of the gel-form film, a weight wascorresponding to the used tetrahydrofuran was confirmed. As a result ofa tensile strength measurement of the gel-form film, a value of 2.22 MPawas obtained.

A ca. 80 μm-thick gel-form polymer electrolyte film was prepared in thesame manner as in Example 4 except for using the polymer obtained inPolymer Preparation Example 5. As a result of evaluation of electrolyticsolution-retentivity of the gel film in the same manner as in Example 4,the percentage weight loss due to exudation was as small as 0.13%, thusexhibiting an excellent electrolytic solution-retentivity.

The above Examples are comparable to Examples 1 and 4 in respects ofpolymer composition and inherent viscosity. Thus, it is understood thatthe polymer of Polymer Preparation Example 5 obtained through a higherpolymerization temperature provided a higher electrolyticsolution-retentivity than the polymer of Polymer Preparation example 1obtained at a lower polymerization temperature. This may be understoodas a result of an increased abnormal linkage content owing to anelevated polymerization temperature.

Example 11

A ca. 100 μm-thick gel-form film containing ca. 75 wt. % of ethylenecarbonate and propylene carbonate was prepared in the same manner as inExample 1 except for using the polymer obtained in Polymer PreparationExample 6. As a result of weighing of the gel-form film, a weight wascorresponding to the used tetrahydrofuran was confirmed. As a result ofa tensile strength measurement of the gel-form film, a value of 1.45 MPawas obtained.

A ca. 80 μm-thick gel-form polymer electrolyte film was prepared in thesame manner as in Example 4 except for using the polymer obtained inPolymer Preparation Example 6. As a result of evaluation of electrolyticsolution-retentivity of the gel film in the same manner as in Example 4,the percentage weight loss due to exudation was as small as 0.10%, thusexhibiting an excellent electrolytic solution-retentivity.

Example 12

A ca. 100 μm-thick gel-form film containing ca. 75 wt. % of ethylenecarbonate and propylene carbonate was prepared in the same manner as inExample 1 except for using the polymer obtained in Polymer PreparationExample 7. As a result of weighing of the gel-form film, a weight wascorresponding to the used tetrahydrofuran was confirmed. As a result ofa tensile strength measurement of the gel-form film, a value of 2.76 MPawas obtained.

A ca. 80 μm-thick gel-form polymer electrolyte film was prepared in thesame manner as in Example 4 except for using the polymer obtained inPolymer Preparation Example 7. As a result of evaluation of electrolyticsolution-retentivity of the gel film in the same manner as in Example 4,the percentage weight loss due to exudation was as small as 0.07%, thusexhibiting an excellent electrolytic solution-retentivity.

INDUSTRIAL APPLICABILITY

As is clear from the above Examples and Comparative Examples, accordingto the present invention, it is possible to obtain a polymer electrolytepresent in a state of containing much nonaqueous electrolytic solutionand exhibiting an excellent strength in this state by using a vinylidenefluoride copolymer having a high vinylidene fluoride content and a highinherent viscosity. Further, by using the polymer electrolyte, it ispossible to obtain a nonaqueous battery having stable strength andproperties and also a high shape latitude.

1-9. (canceled)
 10. A process for producing a polymer electrolyte for anonaqueous battery, comprising: mixing a vinylidene fluoride copolymerand a nonaqueous electrolytic solution with a solvent that can beevaporated, wherein the vinylidene fluoride copolymer comprises 80 to 97wt. % of vinylidene fluoride monomer units and 3 to 20 wt % of units ofat least one monomer copolymerizable with vinylidene fluoride monomerand has an inherent viscosity of 1.5 to 10 dl/g, and evaporating thesolvent to form a polymer electrolyte comprising the vinylidene fluoridecopolymer impregnated with the nonaqueous electrolytic solution.
 11. Aprocess according to claim 10, wherein the vinylidene fluoride copolymerhas an inherent viscosity of 1.7 to 7 dl/g.
 12. A process according toclaim 10, wherein the vinylidene fluoride copolymer is caused to have aninherent viscosity of 1.8 to 7 dl/g in the polymerization.
 13. A processaccording to claim 10, wherein said at least one monomer copolymerizablewith vinylidene fluoride monomer comprises a mixture ofhexafluoropropylene monomer and trifluorochloroethylene monomer.
 14. Aprocess according to claim 10, wherein the polymer electrolyte is causedto contain 50-85 wt. % of the nonaqueous electrolytic solution.
 15. Aprocess according to claim 10, wherein the vinylidene fluoride copolymeris crosslinked.
 16. A process according to claim 15, wherein thevinylidene fluoride copolymer is crosslinked in the presence of (1) acrosslinking agent selected from the group consisting of polyamides,polyols and polymerizable crosslinking agents having an unsaturatedbond, and (2) a radical generating agent.
 17. A process according toclaim 15, wherein the vinylidene fluoride copolymer is crosslinked byirradiation with electron rays or gamma rays.
 18. A non-aqueous battery,comprising: a positive electrode comprising a positive electrodematerial capable of being doped with and liberating lithium, a negativeelectrode material similarly capable of being doped with and liberatinglithium, and a polymer electrolyte produced by a process comprising:introducing 80 to 97 wt. % of vinylidene fluoride monomer and 3 to 20 wt% of at least one monomer copolymerizable with vinylidene fluoridemonomer into a polymerization vessel, all of said vinylidene fluoridemonomer and at least one monomer being introduced simultaneously intothe polymerization vessel before commencing polymerization, thensuspension-polymerizing the monomers to provide a vinylidene fluoridecopolymer comprising polymerized units of the monomers and having aninherent viscosity of 1.5 to 10 dl/g, and impregnating the vinylidenefluoride copolymer with a nonaqueous electrolytic solution to provide apolymer electrolyte.
 19. A non-aqueous battery according to claim 18,wherein the vinylidene fluoride copolymer is caused to have an inherentviscosity of 1.7 to 7 dl/g in the polymerization.
 20. A non-aqueousbattery according to claim 18, wherein the vinylidene fluoride copolymeris caused to have an inherent viscosity of 1.8 to 7 dl/g in thepolymerization.
 21. A non-aqueous battery according to claim 18, whereinsaid at least one monomer copolymerizable with vinylidene fluoridemonomer comprises a mixture of hexafluoropropylene monomer andtrifluorochloroethylene monomer.
 22. A non-aqueous battery according toclaim 18, wherein the polymer electrolyte is caused to contain 50-85 wt.% of the nonaqueous electrolytic solution.
 23. A non-aqueous batteryaccording to claim 18, wherein the process further comprisescrosslinking the vinylidene fluoride copolymer.
 24. A non-aqueousbattery according to claim 23, wherein the vinylidene fluoride copolymeris crosslinked in the presence of (1) a crosslinking agent selected fromthe group consisting of polyamides, polyols and polymerizablecrosslinking agents having an unsaturated bond, and (2) a radicalgenerating agent.
 25. A non-aqueous battery according to claim 23,wherein the vinylidene fluoride copolymer is crosslinked by irradiationwith electron rays or gamma rays.
 26. A non-aqueous battery, comprising:a positive electrode comprising a positive electrode material capable ofbeing doped with and liberating lithium, a negative electrode materialsimilarly capable of being doped with and liberating lithium, and apolymer electrolyte produced according to the process of claim
 10. 27. Anon-aqueous battery, comprising: a positive electrode comprising apositive electrode material capable of being doped with and liberatinglithium, a negative electrode material similarly capable of being dopedwith and liberating lithium, and a polymer electrolyte producedaccording to the process of claim
 11. 28. A non-aqueous battery,comprising: a positive electrode comprising a positive electrodematerial capable of being doped with and liberating lithium, a negativeelectrode material similarly capable of being doped with and liberatinglithium, and a polymer electrolyte produced according to the process ofclaim
 12. 29. A non-aqueous battery, comprising: a positive electrodecomprising a positive electrode material capable of being doped with andliberating lithium, a negative electrode material similarly capable ofbeing doped with and liberating lithium, and a polymer electrolyteproduced according to the process of claim
 13. 30. A non-aqueousbattery, comprising: a positive electrode comprising a positiveelectrode material capable of being doped with and liberating lithium, anegative electrode material similarly capable of being doped with andliberating lithium, and a polymer electrolyte produced according to theprocess of claim
 14. 31. A non-aqueous battery, comprising: a positiveelectrode comprising a positive electrode material capable of beingdoped with and liberating lithium, a negative electrode materialsimilarly capable of being doped with and liberating lithium, and apolymer electrolyte produced according to the process of claim
 15. 32. Anon-aqueous battery, comprising: a positive electrode comprising apositive electrode material capable of being doped with and liberatinglithium, a negative electrode material similarly capable of being dopedwith and liberating lithium, and a polymer electrolyte producedaccording to the process of claim
 16. 33. A non-aqueous battery,comprising: a positive electrode comprising a positive electrodematerial capable of being doped with and liberating lithium, a negativeelectrode material similarly capable of being doped with and liberatinglithium, and a polymer electrolyte produced according to the process ofclaim
 17. 34. A non-aqueous battery according to claim 18, wherein thevinylidene fluoride copolymer is impregnated with the nonaqueouselectrolytic solution by mixing the vinylidene fluoride copolymer andthe nonaqueous electrolytic solution with a solvent that can beevaporated, and evaporating the solvent.